Solid state electron multiplier including reverse-biased, dissimilar semiconductor layers



DISSIMILAR SEMICONDUCTOR LAYERS Filed Dec. 1963 VACUUM F i g. 2.

T. P. BRODY REVERSE-BIASED TO NEXT STAGE Jan. 16, 1968 SOLID STATEELECTRON MULTIPLIER INCLUDING VACUUM DISTANCE WITNESSES United StatesPatent Ofifice 3,364,367 SULlD STATE ELECTRON MULTlPLlIER IN- CLUDENGREVERSE-BIASED, DllSSIMILAR SEMHIONDUCTOR LAYERS Thomas P. Brody,Pittsburgh, Pa., assignor to Westinghouse Electric Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Filed Dec. 12, 1963, Ser.No. 339,053 6 Claims. (Cl. 307-308) The present invention relates toelectron multipliers, and more particularly to electron multipliersincorporated in solid state semiconductor devices.

Many of the solid state electron multipliers have the inherent failingof being of relatively .low yield. That is, the number of electronsactually emitted is low for the number of incident electrons applied tothe device. The reason for the low yields may be explained due tovarious factors such as impurities in the materials used, unjudiciousselection of materials and thicknesses. Even a more fatal defect indevices is that of the mechanism by which the device functions in onlysupplying a limited number of electrons of sufficient energy to escapeand serve a useful function.

It is therefore an object of the present invention to provide a new andimproved electron multiplier.

It is a further object of the present invention to provide a new andimproved electron multiplying device utilizing the characteristics of areverse-biased p-n junction within a semiconductor structure.

Broadly, the present invention provides an electron multiplying devicein which a semiconductor device having a p-n junction therein isirradiated with high energy electrons to generate a large number ofelectron-hole pairs. These pairs are separated .at the p-n junction,which is reverse-biased, and are accelerated towards the surface. Withappropriate construction, a large number of electrons are emitted foreach incident electron.

These and other objects and advantages of the present invention willbecome more apparent when considered in view of the followingspecification and drawings, in which:

FIGURE 1 is a schematic diagram showing one embodiment of the electronmultiplier of the present in vention;

FIG. 2 is an energy diagram demonstrating the operation of theembodiment of FIG. 1; and

FIG. 3 is another embodiment of the electron multiplier of the presentinvention.

Referring to FIG. 1, a single stage of an electron multiplying device isshown. Of course, a plurality of stages may be connected in tandem withthe output elec trons of one stage serving as the input incidentelectrons to the next stage with multiplication occuring in each of thestages.

In FIG. 1, the electron multiplying device has a layer 12 of anelectrically conductive material, such as aluminum or gold. The layer 12has a surface 14 to which incident electrons e of a relatively highenergy level are applied thereto from an external source or from aprevious stage of electron multipliers. The conducting layer 12 is madethin enough so that substantially all of the electrons irradiating thesurface 14 will be transmitted through the layer 12 to an oppositesurface 16 of the layer 12. Disposed next to the conducting layer 12 atthe surface 16 is a semiconductive layer 18. The semiconductive layer 18is of a p-type semiconductivity. An ohmic contact is formed 3,364,367Patented Jan. 16, 1968 at the interface surface 16 between theconducting layer 12 and the semiconductive layer 18 so that electronstransmitted through the layer 12. may penetrate into the p-typesemiconductive material of the layer 18. The layer 12 may be fixed tothe semiconductive layer 18 by evaporation or alloying in order to forman ohmic contact at the interface 16. An n-type semiconductive layer 29joins the ptype semiconductive layer 18. A p-n junction 22 is formedbetween the semiconductive layers 18 and 20. The layers 18 and 2d may beformed from a single wafer with a ptype semiconductive wafer beingditiused with an n-type impurity, or an n-type semiconductive wafer maybe diffused with a p-type impurity. Both of these diffusion methods arewell known in the art. Also epitaxial growth techniques could beutilized in order to form the pa junction 22 between the two types ofsemiconductivities.

The n-type semiconductive layer 20 has a surface 24 disposed continuousto a vacuum area 26. The entire device or a plurality of devices isenclosed within a tube 28, which for example, may be glass.

To reverse bias the p-n junction 22, a biasing source comprising thebattery 30 and a resistor 32 is provided. The negative terminal of thebattery terminal is connected ohmically to the conductive layer 12. Thepositive termi nal of the battery 30 is connected to the resistor 32with the other end of the resistor being connected ohmically to theperiphery of the n-type semiconductive region 20 at the ohmic contact34. The conducting layer 12 also serves as a terminal for a battery 36which may serve as the inter-stage biasing source for subsequent stagesof multiplying devices.

Referring now to FIG. 2, an incident electron a, being of a relativelyhigh energy of the order of 10 lrv., incident upon the surface M of theconducting layer 12 will pass through the thin metal film and penetrateinto the p-type region 18. The thickness of the p-type layer 18 ischosen so that the excess energy of the electron is fully dissipated inthe production of electron-hole pairs. This dissipation is approximately3.7 electron volts per pair. In the energy diagram, the electron holepairs are shown by the small clashes in the conduction band and circlesin the valence band of the p-type region 13. The: pairs diffusing towardthe 13-11 junction 22 are separated by the p-n junction electric field,which is established by the reverse bias of the battery 30. It should benoted that if the semiconductive device layers 18 and 2d are biased inthe avalanche breakdown region, further carrier multiplication may beobtained. Due to the separation of the electronhole pairs, excesselectrons will be transmitted into the n-type region 2% with many of theelectrons being at relatively high energy levels. By selecting thereverse bias on the p-n junction 22 of a sutficient magnitude and makingthe n-type region 2% thin enough, a substantial proportion of the excesselectrons moving toward the surface 24 of the layer .Tztl will havesufficient energy to escape over the vacuum barrier 38 to provide acopious supply of electrons into the evacuated area 26. To lower thevacuum barrier, the surface 24 may be suitably treated, for example,cesiatcd. See, for instance, copending application Ser. No. 217,581,filed Aug. 17, 1962 and assigned to the same assignee as the presentapplication which describes various structures for lowering the vacuumbarrier at vacuum interfaces. The thickness of the n-type layer 20 maybe reduced to a suitable value by a slow etch or by a jet etchingprocess.

With incident electrons having energy levels of the rder of 10 kv., thenumber lof electron-hole pairs generated by a single electron may beseveral thousand. With avalanche multiplication, this factor can beincreased by another order of magnitude. An electron transfer ratio ofat least 0.1 may be expected through the n-type layer. Therefore, it canbe seen th at single stage gain of 1000 or better can be expected by thedevice as shown in FIG. 1.

The n type region is usually highly doped and an ohmic contact is madeat 34 on the periphery of the layer. The highly doped ntype region,however, may excessively, in certain applications, reduce electronmobility. To avoid this problem, a structure such as shown in FIG. 3 maybe utilized.

The embodiment of FIG. 3 is substantially the same as that of FIG. 1with a metallic layer 12 being disposed adjacent a p type semiconductivelayer 18, forming a p-n junction 22 with an n-type semiconductive region20. The difference between the embodiments of FIG. 3 and FIG. 1 lies inthat an n-type region of low conductivity is used and a grid structureis disposed on the surface 24 of the layer 20. The positive terminal ofthe battery 36 is connected through the resistor 32 to the gridstructure 40 rather than at the ohmic contact 34 on the periphery of then type region 29 of FIG. 1. The grid structure til may be formed byevaporating a contact over the surface 24. The device of FIG. 3functions in the same flashion as that of FIG. 1 with the energy diagramof FIG. -12 being equally applicable thereto.

It can thus be seen that an electron multiplying device is provided.Incident electrons pass through a metal layer and penetrate into asemiconductive layer of one type semiconductivity and produceelectron-hole pairs. The pairs drift toward a p-n junction under theeffect of a biasing source. Electrons are separated at the junction toprovide excess electrons which pass through a semiconductive region ofanother type semiconductivity with sufficient energy to be emitted intoa vacuum.

Although the present invention has been described with a certain degreeof particularity, it should be understood that the present disclosurehas been made only by way of example and that numerous changes in thedetails of fabrication, materials used and the combination andarrangement of elements may be resorted to without departing from thescope and the spirit of the present invention.

I claim as my invention:

1. An electron multiplier operative with incident electrons comprising,a first layer comp-rising an electrically conductive materialtransmissive to irradiating electrons, a second layer comprising asemiconductive material of one type of semiconductivity, said secondlayer being disposed adjacent said first layer, said second layerproducing a copious number of electron-hole pairs in response to eachincident electron penetrating said second scmiconductive layer, a thirdlayer comprising a semiconductive material of another type ofsemiconductivity from said second layer, said third layer being adjacentsaid second layer and forming a p-n junction therewith, theelectron-hole pairs being separated at said p-n junction, and biasingmeans to reverse bias said pn junction so that the separated electronsmay pass through said third layer.

2. A solid state electron multiplier operative with incident electronscomprising, a first layer comprising an electrically conductive materialtransmissive to irradiating electrons, a second layer comprising asemiconductive material of one type of semiconductivity, said secondlayer being disposed adjacent said first layer, said second layerproducing a copious number of electron-hole pairs in response to eachincident electron penetrating said second layer, a third layercomprising a semiconductive material of another type of semiconductivityfrom said second layer, said third layer being adjacent said secondlayer and forming a p-n junction therewith, the electronhole pairs beingseparated at said p-n junction, and biasing means connected ohmically tosaid first and third layers to reverse bias said p-n junction betweensaid second and third layers so that the separated electrons may passthrough said third layer and be emitted into vacuum.

3. A solid state electron multiplier operative with in cident electronscomprising, a contact layer comprising an electrically conductivematerial transmissive to irradiating elcctrons, a first semiconductivelayer comprising a p-type semiconductive material, said first layerbeing disposed adjacent said contact layer and forming an ohmic contacttherebetween, said first layer producing a copious number ofelectron-hole pairs in response to each incident electron penetratingsaid first semiconductive layer, a second semiconductive layercomprising an n-type semiconductive material, said second layer beingadjacent said first layer and forming a p-n junction therewith, theelectron-hole pairs being separated at said p-n junction and biasingmeans to reverse bias said p-n junction between said semiconductivelayers so that the separated electrons may pass through said secondlayer and be emitted into Vacuum.

4. A solid state electron multiplier operative with incident electronscomprising, a contact layer comprising an electrically conductivematerial transmissive to irradiating electrons, a first semiconductivelayer comprising a p-type semiconductive material, said first layerbeing disposed adjacent said contact layer and forming an ohmic contacttherebetween, said first layer producing a copious number ofelectron-hole pairs in response to each incident electron penetratingsaid second semiconductive layer, a second semiconductive layercomprising an n-type semi conductive material, said second layer beingadjacent said first layer and forming a p-n junction therewith, theelectron-hole pairs being separated at said p-n junction, and biasingmeans connected ohmically to said contact layer and said second layer toreverse bias said p-n junction between said semiconductive layers sothat the separated electrons may pass through said second layer and beemitted into vacuum.

5. An electron multiplier operative with incident electrons comprising,a first contact layer comprising an electrically conductive materialtransmissive to irradiating electrons, a second layer comprising asemiconductive material of one type of semiconductivity, said secondlayer being disposed adjacent said first layer, said second layerproducing a copious number of electron-hole pairs in response to eachincident electron penetrating said second semiconductive layer, a thirdlayer comprising a semiconductive material of another type ofsemiconductivity from said second semiconductive layer, said third layerbeing adjacent said second layer and forming a p-n junction therewith,the electron-hole pairs being separated at said p-n junction, a gridmatrix disposed adjacent said third layer, and biasing means connectedto said first layer and said grid matrix to reverse bias said p-njunction between said second and third layers so that the separatedelectrons may pass through said third layer and said grid matrix and beemitted into vacuum.

6. A solid state electron multiplier operative with incident electronscomprising, a first layer comprising an electrically conductive materialtransmissive to irradiating electrons, a second layer comprising ap-type semiconductive material, said second layer being disposedadjacent said first layer and forming an ohmic contact therebetween,said second layer producing a copious number of electron-hole pairs inresponse to each incident electron penetrating said second layer, athird layer comprising an n-type semiconductive material, said thirdlayer being adjacent said second layer and forming a p-n junctiontherewith, the electron-hole pairs being separated at said p-n junction,a grid matrix disposed adjacent said third layer and having openingstherein to permit the passage of electrons, and biasing means connectedto said first layer and said grid matrix to reverse bias said p-n junc-5 tion between said second and third layers so that the sepa- 2,970,219rated electrons may pass through said third layer and said 3,036,234grid matrix and be emitted into vacuum. 3,098,168 3,105,166 ReferencesCited 5 3,119,947 UNITED STATES PATENTS Roberts et a1. 313-65 Dacey328-243 Aigrain 313-346 Choyke et a1 313-34-6 Goetzberger 317-234 ROBERTSEGAL, Primary Examiner.

8/1959 Sternglass et a1 313-103 11/1960 Burton 313-346 JAMES W.LAWRENCE, Examiner.

1. AN ELECTRON MULTIPLIER OPERATIVE WITH INCIDENT ELECTRONS COMPRISING,A FIRST LAYER COMPRISING AN ELECTRICALLY CONDUCTIVE MATERIALTRANSMISSIVE TO IRRADIATING ELECTRONS, A SECOND LAYER COMPRISING ASEMICONDUCTIVE MATERIAL OF ONE TYPE OF SEMICONDUCTIVELY, SAID SECONDLAYER BEING DISPOSED ADJACENT SAID FIRST LAYER, SAID SECOND LAYERPRODUCING A COPIOUS NUMBER OF ELECTRON-HOLE PAIRS IN RESPONSE TO EACHINCIDENT ELECTRON PENETRATING SAID SECOND SEMICONDUCTIVE LAYER, A THIRDLAYER COMPRISING A SEMICONDUCTIVE MATERIAL OF ANOTHER TYPE OFSEMICONDUCTIVITY FROM SAID SECOND LAYER, SAID THIRD LAYER BEING ADJACENTSAID SECOND LAYER AND FORMING A P-N JUNCTION THEREWITH, THEELECTRON-HOLE PAIRS BEING SEPARATED AT SAID P-N JUNCTION, AND BIASINGMEANS TO REVERSE BIAS SAID P-N JUNCTION SO THAT THE SEPARATED ELECTRONSMAY PASS THROUGH SAID THIRD LAYER.